Computing devices, such as notebook computers, personal data assistants (PDAs), kiosks, and mobile handsets, have user interface devices, which are also known as human interface devices (HID). One such user interface device is a remote control having multiple buttons for controlling a separate device, such as a television (TV), video cassette recorder (VCR), digital video recorder (DVR), digital video disc (DVD) player receiver, computer, radio, lights, fans, industrial equipment, or the like. Conventional remote controls, however, are limited to having mechanical buttons.
Capacitance sensing is used in wide variety of user interface applications. Examples include touchpads on notebook computers, touchscreens, slider controls used for menu navigation in cellular phones, personal music players, and other hand held electronic devices. One type of capacitance touch-sensor device operates by way of capacitance sensing utilizing capacitive sensors. The capacitance detected by a capacitive sensor changes as a function of the proximity of a conductive object to the sensor. The conductive object can be, for example, a stylus or a user's finger. The touch-sensor devices may include single sensor elements or elements arranged in multiple dimensions for detecting a presence of the conductive object on the touch-sensor device. Regardless of the method, usually an electrical signal representative of the capacitance detected by each capacitive sensor is processed by a processing device, which in turn produces electrical or optical signals representative of activation, position, or the like of the conductive object in relation to the touch-sensor device, such as a touch-sensor pad. A touch-sensor strip, slider, or button operates on the same capacitance-sensing principle.
Capacitance sensing has been implemented in a wide variety of electronic devices to replace mechanical buttons in the electronic devices. Capacitance sensing has many advantages over conventional cursor control devices, mechanical switches, and rotary encoders. A principal such advantage is the lack of moving parts, which allows capacitance sensing to provide great improvements in reliability since there are no moving parts to wear out.
Typically, a capacitance sensing system detects changes in capacitance between a sensing element and electrical ground. For example, in a cap sense button, when the users' finger is in close proximity to the sensor element, a capacitance is formed between the sensor element and the finger—and as the finger is effectively at a ground potential, a detectable capacitance to ground is present when the user's finger is close to the sensor element. In a touchpad or slider, the position of the user's finger is detected by measuring the difference in capacitance to ground between a number of sensing elements arranged as an array.
One disadvantage of capacitance sensing systems is that the capacitance sensing systems sometimes consume more power in the “sleep” mode than do their mechanical equivalents. For example, consider a remote control such as a typical audio-visual remote control used to control a TV, VCR, DVD, etc. Typically, such a device may have 40 buttons, including the digits 0-9, volume up/down, channel up/down, etc. In a conventional button implementation, the mechanical switches would be arranged as an array of eight by five (8×5) buttons, with “scan” and “sense” rows and columns of switches. When no button is pressed, the remote control is in the lowest possible power-consumption state in order to maximize battery life. In this state, typically all the row connections are connected via a resistor to a high voltage and all the column connections are connected to ground. When a button is pressed, one or more of the row connections are pulled to ground through the switch, causing current to flow through the resistor(s). This then wakes up the controller IC which begins scanning the rows and columns to determine which button has been pressed. However, when no button is pressed, there is no current flow at all.
FIG. 1A illustrates a block diagram of a conventional remote control 100 having multiple buttons. The multiple buttons are implemented using a key matrix 102. The key matrix 102, as described in more detail with respect to FIGS. 1B, 1C, and 1D, is coupled to a microcontroller unit (MCU) 101. The MCU 101 is coupled to power circuitry that may include a power source, such as one or more batteries, a power transistor, and a diode. The MCU 101 may be an embedded controller that performs a variety of tasks, all of which help to cut down on the overall system overhead. The MCU 101 may monitor the buttons and report to the main computer whenever a button is pressed or released.
FIG. 1B illustrates a conventional key matrix 102 of FIG. 1. The conventional key matrix 102 includes multiple rows (X0-X2) 101(0)-101(2), and multiple columns (Y0-Y2) 102(0)-102(2). All the rows 101(0)-101(2) are each connected to a pull-up resistor (e.g., 103(0)-103(2)), and all the columns 102(0)-102(2) are each connected to a pull-down transistor (e.g., 104(0)-104(2)), such as an N-Channel MOSFET. Above the key matrix 102 there are multiple buttons 105(0)-105(8). Upon pressing a button, the corresponding row and column (X, Y) are shorted together. For example, the row X will read “0,” otherwise the row X is “1.” Each button sits over two isolated contacts of its corresponding row and column in the scan matrix. When a button is pressed, the two contacts are shorted together, and the row and column of the button are electrically connected.
FIG. 1C illustrates scan results for no buttons pressed on a conventional key matrix. The controller writes a scan pattern 109 out to the column lines consisting of all “1”s and one “0” which is shifted through each column. In FIG. 1C no buttons are pressed, resulting in all “1”s in the scan results 110 being read at the row lines. FIG. 1D illustrates scan results for a button 111 pressed on a conventional key matrix. The controller writes a scan pattern 112 out to the column lines consisting of all “1”s and one “0” which is shifted through each column. The scan results 113 are then read at the row lines. If a “0” is propagated to a row line, then the button 111 at the intersection of that column and row has been pressed.
However, in a typical capacitance sensing implementation of the same remote control, the controller must wake periodically (e.g., typically every 100 milliseconds (ms)) and measure the capacitance on each button in order to detect whether or not a finger is present. Typically, such a measurement may take 250 microseconds (μs) including the time taken to make the actual measurement and the associated processing time. Therefore, in a 40-button system it may take 10 ms to detect whether or not a button has been pressed. If the controller consumes 2.5 milliamps (mA) while performing such scanning, the average current consumption of the device when no button is pressed is approximately 250 micro amps (μA). Such a high “sleep” current is regarded as being unacceptable by most remote control manufacturers because of the resulting short battery life, and capacitance sensing has therefore not been widely used in remote controls and other battery-powered devices.
Similar high sleep current is present in track pads and sliders, but the applications are often more tolerant of a high sleep current, because of the usage models of such devices. Even in these applications, a reduction in sleep current would still be beneficial.
Using capacitance sensing in remote controls may result in high “sleep” current when no finger is present on the device in order to be able to quickly detect the presence of a finger when that occurs. Also, the key matrices cannot be built in very small areas because it is limited by the pull-up resistor and mechanical button for each button. For example, the mechanical button of each button may have an area of about 0.5 centimeters (cm)×0.5 cm.